In New York, anything can happen. When it involves crime, NYPD steps in. Det. Mac Taylor (Gary Sinise) is enjoying one of his rare days off when his romantic interest, Dr. Peyton Driscoll (Claire Forlani), gets a call-out for a medical examiner at a crime scene. Mac decides to ride along.
They arrive to find Det. Danny Messer (Carmine Giovinazzo) wondering if a dead woman in a bathtub might have been murdered. The only evidence supporting his theory is a bullet shard lying nearby. Peyton examines the body, only to discover no exit wound. This means any bullet which entered the body never left.
Mac notices a 'universal key', or credit card, which he believes the woman used to open the door. It's a simple matter of sliding plastic between the door frame and deadbolt, and entry is easy. Before Mac heads out to a new investigation, he surprises Danny with the news of his involvement with Peyton.
It's taken Mac a long time to get back in the dating field after his wife died, but it appears to be working. When Mac said, “We're together", it was the first time he acknowledged his personal life to a member of his team. He once mentioned to Dr. Sid Hammerback (Robert Joy) he had plans for Thanksgiving, but did not say with whom. Mac agreed Danny was the last to know he was in a new relationship.
Across town, Det. Don Flack (Eddie Cahill) tells Mac the place where their latest victim, a music mogul, died is well known to the cops. Twice before, a break-in by an unknown female was cause for an incident report. The victim's sister, Kennedy, has kept this fact well in mind, and has a low opinion of the department for not following through.
Mac talks to her, and tries to get the full story about the attack on her, and her brother's death. Det. Stella Bonasera (Melina Kanakaredes) processes evidence. A piece of glass from the shattered aquarium cuts into her arm, and she goes to wash off the blood.
At the morgue, Sid gives Stella a nasty shock. The victim was HIV positive, which puts her at risk. Just to be on the safe side, Stella asks Adam Ross (A.J. Buckley), one of the lab workers, if DNA found on the glass was only from the victim. He confirms it, which sends Stella to a testing facility for blood work. She will have to wait three months to know for sure if she has the precursor to AIDS.